PURPOSE: To prevent effectively the generation of resin burrs at a heat sink part by a method wherein the clearances between lugging punches and a knockout for pressing the heat sink are made large.

CONSTITUTION: Clearances A between a knockout 6 and lugged punches 7 are set at a value of 20μm or thereabouts to perform a lugging work by a press processing. Thereby, when a heat sink 2 is subjected to press processing and lug parts 5 are formed, a material is formed into a risen form at the edge parts of step parts 5a and projected edge parts 10 are formed. The height of the parts 10 is formed in 2 to 4μm. In respect to the parts, on which the parts 10 are formed, of the exposed surface of the sink 2, a resin molding is conducted. Thereby, run of a molding resin to the back is effectively suppressed and the generation of resin burrs can be effectively prevented.